1. Field of the Invention
The present invention relates to a method of forming a metal plug in the manufacture of a semiconductor device and to a wafer processing apparatus used for forming the metal plug.
2. Description of the Prior Art
As a semiconductor device is micropatterned, the area of a contact hole is decreased, but the thickness of an interlayer insulator cannot be decreased because a high breakdown voltage must be assured. For this reason, the aspect ratio of the contact hole tends to be increased. Therefore, when a wiring layer consists of only aluminum, a disconnection easily occurs at a contact step portion due to the poor step coverage of aluminum, and the reliability of the semiconductor device is degraded.
In contrast to this, a method in which, after a contact hole is formed, a polysilicon film is deposited on the entire surface of the resultant structure, and the polysilicon film is etched back to leave the polysilicon film in only the contact hole is proposed. A so-called selective W-CVD method in which a W film is selectively formed in only the contact hole by using the reduction reaction of WF6 is proposed (for example, Japanese Patent Laid-Open No. 62-229959).
Although the method of burying the contact with the polysilicon film can be realized by a prior art extension, the resistance of polysilicon itself is higher than that of a metal. In the selective W-CVD method, it is difficult to always obtain complete selectivity, and the theoretical problem that contact holes having different depths cannot be simultaneously buried with the W film is left.
A so-called blanket W-CVD method is proposed (for example, Japanese Patent Laid-Open No. 62-229959). In this blanket W-CVD method, after a contact hole is formed, a W film is deposited on the entire surface of the resultant structure, and the W film is etched back to leave the W film in only the contact hole. For this reason, the W film can be formed easier than that in the selective W-CVD method, and contact holes having different depths can be simultaneously buried with the W film.
In this blanket W-CVD method, even when a TiON layer is formed to improve the adhesion properties with the SiO2 film serving as an insulating film, the contact holes can be buried with the W film. This TiON layer also functions as a barrier layer, and the melting point of W itself is high, i.e., 3,380xc2x0 C. For this reason, even when the W film is formed at a relative high temperature, permeation of W into the Si substrate can be suppressed, thereby obtaining preferable electrical characteristics.
FIGS. 1A to 1C show a conventional method of forming a W plug using the above blanket W-CVD method and an etch-back technique. As shown in FIG. 1A as a state before a W plug is formed, a diffused layer 12 is formed in an Si substrate 11, and an SiO2 film 13 serving as an interlayer insulator is formed on the Si substrate 11.
In this prior art, a contact hole 14 is formed in the SiO2 film 13 to electrically connect with the diffused layer 12. For this purpose, a resist film (not shown) having an opening corresponding to the contact hole 14 is formed on the SiO2 film 13 by photolithography, and etching is performed using the resist film as a mask by an RIE apparatus at a reaction gas flow rate of O2/CHF3=8/75 SCCM, a reaction pressure of 50 mTorr and an RF power of 1 kW.
As shown in FIG. 1B, a TiON layer 15 serving as a titanium-based material layer for improving the adhesion properties between the SiO2 film 13 and a W film (to be formed later) is formed on the entire surface of the resultant structure by reactive sputtering. As the titanium-based material layer, although a TiN layer or the like may be used, a TiON layer is preferably used. Thereafter, a W film 16 is deposited on the entire surface of the resultant structure by using, e.g., a cold-wall type CVD apparatus, at a reaction temperature of 400xc2x0 C., a reaction pressure of 6.5 Torr and a reaction gas flow rate ratio of H2/WF6=1/19.
The entire surface of the W film 16 is etched back with a gas such as SF6 containing fluorine, as shown in FIG. 1C, the W film 16 is left in only the contact hole 14, and the W film 16 is used as a plug. At this time, when the TiON layer 15 is etched simultaneously with the W film 16, etching back is preferably performed with a gas obtained by adding a gas such as Cl2 containing chlorine to the gas containing fluorine.
As shown in FIG. 1B, however, the surface of the W film 16 has an uneven shape at the time the W film 16 is deposited. For this reason, as shown in FIG. 1C, the uneven shape is transferred to the SiO2 film 13. This transfer makes it impossible to form a high-quality wiring layer on the SiO2 film 13. In addition, since the surface of the W film 16 left in the contact hole 14 is kept uneven, a wiring layer formed on the SiO2 film 13 cannot preferably be in electrical contact with the W film 16.
When a gas such as SF6 containing fluorine is used such that ion species are not mainly used in etching but a radical reaction is mainly used in the etching, the uneven shape is prevented from being transferred to the SiO2 film 13, and an etching rate is increased. However, a loading effect during etching is increased.
In over-etching after just-etching is performed to expose part of the SiO2 film 13, since the etching area of the W film 16 is abruptly decreased, an etching rate of the W film 16 in the contact hole 14 is abruptly increased. For this reason, the over-etching of the W film 16 cannot be easily controlled, and as shown in FIG. 1C, the contact hole 14 is buried or filled in a recessed form.
In addition, as shown in FIG. 1C, a recessed portion 17 may be formed at the central portion of the W film 16 in the contact hole 14 because of the following reason. That is, the W film 16 is formed from the bottom surface and side surface of the contact hole 14 during the CVD, a seam 18 is formed at the central portion as shown in FIG. 1B, and the W film 16 at the seam 18 is not rigid and is brittle, thereby increasing the etching rate of the W film 16.
When the recessed portion 17 is present, the wiring layer formed on the SiO2 film 13 cannot be in proper electrical contact with the W film 16. For this reason, in the prior art shown in FIGS. 1A to 1C, the reliability of a semiconductor device cannot be improved.
According to the present invention, a method of forming a metal plug, having the steps of forming a contact hole in an insulating film, depositing a metal film on the insulating film, and etching the metal film to bury the contact hole with the metal film, comprises the steps of forming a smoothing layer on the metal layer and the step of etching the smoothing layer and the metal layer as the step of etching the metal film.
As the smoothing layer, a W film formed by bias-sputtering, an SiNx (x=1 to 2) formed by competitive reactions as etching and deposition reactions, a resist film, an SOG film, an organic polymer layer or the like may be used.
In the method of forming a metal plug according to the present invention, the contact hole may be formed in a tapered shape.
A wafer processing apparatus according to the present invention comprises an organic polymer layer forming unit for forming an organic polymer layer and a heating unit for heating to fluidize the organic polymer layer to obtain a smooth organic polymer layer.
According to the present invention, a method of forming a metal plug, having the steps of forming a contact hole in an insulating film, depositing a metal film on the insulating film, and etching the metal film to bury the contact hole with the metal film, comprises the steps of performing first etching by a radical reaction as the step of etching the metal film and then performing second etching in which deposition and etching reactions of the metal layer are competitive reactions.
In the method of forming a metal plug according to the present invention, since the smoothing layer is formed on the metal layer, even when the surface of the metal film has an uneven shape at the time the metal film is deposited on the insulating film, the surface of the metal film can be smoothed during the etching by equalizing the etching rate of the smoothing layer to that of the metal film. For this reason, the uneven shape of the surface of the metal film at the time the metal film is deposited can be prevented from being transferred to the insulating film, and the contact hole can be buried with the metal plug having a smooth surface.
In addition, when the contact hole is formed in a tapered shape, a tapered recessed portion can be formed in the metal film in the contact hole. For this reason, when a brittle seam is formed such that metal layer portions formed from the bottom surface and side surface of the contact hole are brought into contact with each other, the tapered recessed portion is formed on the seam to bury the tapered recessed portion with a thick smoothing layer. Even when the metal layer has a thickness such that metal film portions are not brought into contact with each other, the tapered recessed portion can easily be buried with the smoothing layer.
Therefore, in etching the smoothing layer and the metal film, the brittle seam portion of the metal film can be prevented from being etched at an excessively high rate, and the tapered recessed portion of the metal film can be prevented from being further etched, thereby preventing the recessed portion from being formed in the metal film in the contact hole.
The wafer processing apparatus according to the present invention comprises the organic polymer layer forming unit and the heating unit. For this reason, the formation of the organic polymer layer and smoothing of the organic polymer layer performed by fluidization can be continuously performed.
In the method of forming a metal plug according to the present invention, the first etching is performed by a radical reaction as etching to the deposited metal film. The first etching has a high etching rate.
When the second etching in which the deposition and etching reactions of the metal film are competitive reactions is performed after the first etching is performed, the over-etching of the metal film can easily be controlled because a loading effect is small in the second etching. In addition, even when the surface of the metal layer has an uneven shape at the time the metal layer is deposited on the insulating film, the surface of the metal film can be smoothed with a deposition component. For this reason, the uneven shape of the surface of the metal film at the time the metal film is deposited can be prevented from being transferred to the insulating film, and the contact hole can be buried with a metal plug having a smooth surface.